In digital cordless communications systems which are based on the Bluetooth Standard Version 1.1, the data is transmitted at a rate of about 1 Mbit/s. A two-value GFSK (Gaussian Frequency Shift Keying) modulation method is used in this case. The GFSK modulation method is a frequency-keying modulation method (FSK—Frequency Shift Keying). Furthermore, a Gaussian filter is used at the transmission end in order to limit the frequency bandwidth for GFSK-based modulation. A filter such as this carries out pulse shaping of the frequency or data pulses, with the pulse for each data symbol extending over a time of more than one symbol time period T.
In order to achieve higher data transmission rates, one option is to use modulation methods with more values, such as the four-value DQPSK method (Differential Quadrature Phase Shift Keying) or, in general, the DMPSK method, in which an M-value symbol where M≧4 is transmitted instead of a two-value bit. Future versions of the Bluetooth Standard (possibly even from Version 1.2, but at the latest from Version 2.0) are planning on the data rate being increased by using modulation methods with more values.
In order to increase the data rate in later versions of a standard in standardized digital radio transmission systems, it is worthwhile changing from a modulation method with a small number of values (for example GFSK) to a modulation method with more values (for example DQPSK) once the radio link has been in existence for a certain time. This allows backward compatibility of the new versions of the Standard to the earlier versions of the Standard. The process of setting up a connection, or of setting up a so-called pico network in the case of the Bluetooth Standard, can in this case be carried out by means of the modulation method with a small number of values, which is used for all the appliances to that Standard. If both the appliances in a link that has been set up or in the pico network are designed for modulation with more values, they can be used for the subsequent data transmission.
In general, in digital TDMA (Time Division Multiple Access)-based mobile radio systems, the information is transmitted in the form of data bursts whose timings are defined. In the case of packet-oriented mobile radio systems, a data packet to be transmitted extends over one or more data bursts. A data burst comprises a first data burst header or data packet header. The header contains information required for addressing the remote location and for indication of the packet type, and should thus, for compatibility reasons, be transmitted using a modulation method with a small number of values for all versions of the Standard. In particular, it is also feasible for the header to be indicated to the respective remote location by switching to a second modulation method, which uses more values. Switching to a modulation method which uses more values is then carried out only in a second part of the data burst. If a plurality of data packets are transmitted in succession, the modulation method is thus switched alternately a number of times.
One fundamental problem with wireless communications systems is the frequency offset between the transmitter and the receiver, that is to say an error between the carrier frequency of the received signal and the frequency applied to the mixer in the receiver in order to down-mix the received signal. This may mean either the frequency which is supplied to a single mixer for direct down-mixing to baseband or else the frequency which is supplied to a first mixer for down-mixing to an intermediate frequency and the frequency which is supplied to a second mixer for down-mixing from the intermediate frequency to baseband.
In order to overcome this problem, the frequency offset must be estimated and corrected at the receiver end. In particular, wireless communications systems such as Bluetooth or DECT require a simple solution in terms of the implementation complexity and the power consumption, since the manufacturers are subject to stringent requirements for low costs and low power consumption at the same time. Receiving appliances for cordless communications systems preferably use low-cost crystal oscillators with a relative accuracy of typically 20 ppm. For a Bluetooth communications system, this means a frequency offset in one of two communication partners of 50 kHz. Since the frequency offset can also occur with an opposite mathematical sign in the two communication partners, that is to say the transmitter and the receiver, the maximum frequency offset may be about 100 kHz. Thus, in order to ensure good reception quality, it is absolutely essential to estimate and compensate for the frequency offset in the receiver.
FIG. 1a illustrates the structure of a data burst which can be transmitted by radio in a Bluetooth transmission system based on a Bluetooth Standard higher than 1.1 between the subscribers in a pico network, which has been previously set-up. In FIG. 1a, the data burst or the data packet has an access code which is arranged at the start, has a time duration of 72 μs and comprises a 4 μs-long preamble, a 64 μs-long synchronization word and a 4 μs-long trailer. The access code is modulated using the two-value GFSK modulation method. Identification and synchronization information for the pico network is sent on a standard-specific basis by means of the access code.
In this example, the data is sent at a first data rate of 1 Mbit/s. The access code is followed by a header with a time duration of 52 μs, which is likewise modulated using the two-value GFSK modulation method. In addition to addressing information and details relating to the packet type being used, the header can also contain information about a second data rate which is intended for transmitting subsequent payload data. The header is followed by a section which is formed from an optional, 5 μs-long guard time interval and an 11 μs-long synchronization or training sequence. No data is transmitted during the optional time period for the guard time interval. The guard time interval is used for switching of the modulation-dependent components at the transmission and reception ends.
The synchronization or training sequence has a sequence of training symbols which are known to the receiver and are used for channel estimation. This training sequence is followed by the payload data area. This is transmitted using a second modulation method, based on DMPSK modulation, where M≧4. The payload data end is then followed by a trailer that also ends the data burst.
FIG. 1b illustrates in schematic form the tolerance requirements for the frequency offset over the data burst. According to the figure, the maximum error from a nominal carrier frequency FC during the access code is ±75 kHz. This value relates to the initial frequency offset, including any drift that may occur during the time period of the access code. Any drift that occurs after an initial frequency offset and throughout the rest of the data burst should not exceed ±10 kHz.
The receiver architectures which are known in the prior art, use a frequency offset compensation circuit which is used to set the reference frequency which is emitted from the crystal oscillator, based on an estimated value of the frequency offset. However, this is disadvantageous as additional hardware is required and a relatively long time is required for adjustment of the crystal oscillator.